Process for generating electric power

ABSTRACT

Electrical energy is produced from solid fuels in an ACFBC steam generator with two gas turbines and a steam turbine. The fuel is pyrolyzed in a fluid pyrolysis bed (34) with a hot partial flow of the bed material from the ACFBC steam generator (30), which together with the formed pyro coke is returned to the generator (30) as a fuel. The formed pyro gas (35) is combusted in one of said gas turbines (10), the exhaust gas (16) from which is used for steam generation (17). The other gas turbine (20) is operated with compressed air from its high pressure compressor (22) partially heated in a heat exchanger (51) in a fluid bed (50) with a controlled partial flow of hot bed material from said generator (30). The compressed air is finally heated by burning (24) fuel directly therein, and the exhaust gas from the power turbine part (22) is used as combustion air in the generator (30) which produces super heated high pressure steam for the steam turbine (40).

The presently most environment-friendly process for generating electricpower based on the combustion of solid fossile fuels, such as coal,comprises burning the fuel in a finely divided form in a steam generatorof the type ACFBC (atmospheric circulating fluid bed combustion) withthe addition of finely divided lime stone and/or dolomite. Since it ispossible to carry out ACFBC with >99% of fuel efficiency at such a lowtemperature as 850° C. the NOx formation is very low andsimultaneously >90% of the content of sulphur in the fuel can be bondedto the lime formed by the combustion.

By operating the steam generator at a high pressure, such as 145 bars,and a steam temperature of 535° C. electric power can be generated in acondensor steam turbine with a net efficiency of about 38%.

In order to achieve a higher efficiency, concepts have been developedbased on combinations of gas and steam turbines with combustion in asteam generator under pressure, so-called PFBC (pressurized fluid bedcombustion). The flue gases from the pressurized fluid bed are aftermulti step cyclone cleaning used directly in a gas turbine, and theexhaust gases from said turbine are used for generating a certainquantity of steam. The main steam generation is, however, performed intube panels arranged in the fluid bed and its walls. With a PFBC conceptit is possible to reach with condensation an actual net efficiency ofabout 42%, whereby about 20% of the electric power is generated in thegas turbine.

The drawbacks of a PFBC concept are partly a comparatively higher bedtemperature (880° C.) which means a higher NOx production, and partly arisk for blade failure in the gas turbine in case the cyclone cleaningfails, and a substantial complication of the operation because of theunavailability for inspection and repair caused by enclosing the fluidbed and cyclones in the pressure chamber.

The instant invention constitutes an environment-friendly method forproducing electric power with a net efficiency of about 46% based oncombustion of solid fossile fuels in a steam generator of the type ACFBCin combination with two gas turbines and a condensor steam turbine. Theconcept according to this invention thus gives a substantially highernet efficiency than the PFBC concept although it is less complicatedfrom a mechanical as well as an operational point of view.

The method according to this invention is characterized in that thefossile fuel, which suitably comprise >20% and preferably >30% ofvolatiles, is first pyrolyzed in a finely comminuted form in contactwith hot bed material in a fluid bed through which flows a controlledpartial flow of the circulating bed material flow f an ACFBC steamgenerator, which partial flow combined with formed char is recirculatedto the combustion chamber of the steam generator where the char is usedas fuel, whereas the produced pyro gas, after per se known cleaning, isused in a gas turbine unit, and the exhaust gas from said unit is used,suitably after reheating, for generating steam, preferably high pressuresteam, whereas the other gas turbine unit is operated with compressedair from the high pressure compressor of the unit, which air is heatedindirectly in a heat exchanger submerged in a classical type fluid bedthrough which flows a controlled partial flow of the circulating bedmaterial flow of the ACFBC steam generator, the final heating of saidcompressed air being performed by burning directly therein a gaseousand/or liquid fuel, suitably internally produced such a fuel, especiallypyro gas, and wherein said gas turbine unit is operated with backpressure by using the exhaust gas from the power turbine part thereof,which has a comparatively high temperature, as fluidizing gas (primaryair) and secondary combustion air in the ACFBC steam generator whenproducing there superheated high pressure steam for the condensor steamturbine unit.

The invention is also characterized by performing the separation of thebed material from the hot flue gas suspension, usually comprising 10-80kg/m³ of bed material, which leaves the top of the combustion chamber ofthe ACFBC steam generator, in two steps: a "coarse separation step"wherein the content of bed material in the suspension is decreased with60-90%, and a "fine separation step" wherein the content is reduced to<3% of the original content of bed material in the suspension, whereinthe material from the "coarse separation step" is introduced under thebed surface in the fluid bed with the heat exchanger and is returnedthrough an overflow to the lower part of the combustion chamber of thesteam generator, and wherein the fluid bed in question is fluidized at0.6<ε<0.7 (ε=void volume/total volume) with the aid of a small partialflow of the exhaust gas flow from the power turbine part of the backpressure gas turbine, whereas the material from the "fine separationstep" is fed to the fluid pyrolysis bed, suitably at the bottom thereof,and the mixture of formed char and bed material is recirculated throughan overflow to the lower part of the combustion chamber of the steamgenerator.

The pyrolysis of the finely-divided fossile fuel is according to theinvention performed by blowing the fuel into the lower part of the fluidpyrolysis bed, preferably at several points, said blowing beingperformed with the aid of a gas which is essentially inert against thefuel, and simultaneously the raw pyro gas formed in the bed fluidizesthe bed and is removed from the top of the bed chamber and is subjectedto coarse cleaning in a hot cyclone. From the hot cyclone the gas ispassed to per se previously known cleaning devices where dust and tarare removed, e.g. by washing. The recovered tar etc. is suitably used asa fuel in the combustion chamber of the steam generator and for thereheating of the exhaust gas from said gas turbine unit.

An example of the invention is disclosed below with reference to FIG. 1as a detailed illustration thereof.

This example is related to a power generating plant with a net effect of130 MW based on coal containing 37.0% of volatiles and with an effectiveheat value of 7800 Kcal/kg of dry raw coal and with the followinganalysis:

    ______________________________________                                        C              H     O          N   S                                         ______________________________________                                        %      79.3        5.4   8.9      1.5 0.6                                     ______________________________________                                    

The ash content of the sample was 4.3%.

In a standard coal powder mill apparatus (not disclosed on FIG. 1) 9.1kg/s of dry raw coal are grinded to a grain size of <0.1 mm. Drying sperformed during grinding, suitably with exhaust gases from the exhaustgas boiler (waste heat boiler) 17. The coal powder 60 is injected withthe aid of cleaned pyro gas 38 in the lower part of the pyrolysis beds34, one for each of the hot cyclones 33 which are connected in parallel.The pyrolysis beds 34 have the shape of an upside down truncated cone(venturi bed) and are at the bottom supplied with controlled partialflows of hot bed material (about 835° C.) which has been separated fromthe bed material circuit of the ACFBC steam generator in the cyclones33. Said partial flows are returned to the lower part of the combustionchamber 31 of the steam generator 30 through an overflow in the upperpart of the pyrolysis beds. The bed material in the pyrolysis beds 34 isstirred from the bottom with the aid of small gas flows, suitably withcleaned pyro gas 38 which is supplied so that upward streams are formedin the central parts of the bed and downward streams along the bed wall.Fluidizing per se is caused by the raw pyro gas evolved when the coalgets into contact with the bed material.

In contact with the hot bed material the finely-divided dry coal ispyrolyzed very quickly and converted to char and raw pyro gas which isremoved from the top of the beds 34. The raw gas is to begin with freedfrom the main part of the entrained bed material in the hot cyclones(not shown in FIG. 1) and is thereafter cooled and washed free from dustand tar 36 in a way known per se from cokery technology. The mainfraction of the tar 37 with a minor content of bed material is used asfuel in the combustion shaft 31 of the steam generator 30 whereas a purefraction 39 is used for reheating the exhaust gas 16 from the gasturbine unit 10 prior to the use thereof in a waste heat boiler 17producing saturated high pressure steam.

The main part (about 84%) of the cleaned pyro gas 38 is used as a fuelin the standard gas turbine unit 10 where it is burnt in the combustionchamber 13 at a pressure of 12 bars in 92 kg/sec. of compressed air fromthe high pressure compressor 12. The generated hot (900° C.) gases fromthe combustion chamber 13 expand in the two steps of the turbine 14which drives the high and low pressure compressors 12 and 11 resp. Theexhaust gas from the turbine 12 drives the power turbine 15 which isconnected to the generator 18 with an output of 16,400 kWe.

The exhaust gases 16 from the power turbine 15 have a temperature ofabout 360° C. and must, for the most economical use, be reheated toabout 450° C. which makes possible the use in a waste heat boiler 17 inwhich it is then possible to generate saturated high pressure steam of145 bars and 338° C., which is super heated in the super heater of theACFBC steam generator. 30% of the tar separated in the cleaning device36 is used up for said reheating.

The minor part of the pyro gas (about 14%) is used as fuel for themodified gas turbine unit 20, in the combustion chamber 24 of which itis used for final heating of the hot pressurized air from the heatexchanger 51 to 900° C. In the heat exchanger 51 in the classicalfluidized bed 50 the compressed air from the high pressure compressor 25(11 bars, 325° C.) is heated indirectly to about 800° C. Through thefluidized bed 50 flows a controlled flow of hot (about 840° C.) bedmaterial which has been branched off from the recirculation circuit ofthe ACFBC steam generator 30 in the "coarse step" 32 and is reintroducedinto the fluid bed 50 close to the bottom thereof and is recirculatedthrough an overflow to the lower part of the combustion chamber 31 ofthe steam generator 30.

The hot pressure gas of the combustion chamber 24 is depressurized inthe two steps of the turbine 23, of which the first step drives the highpressure compressor 25 and the second step through a gear 26 drives thegenerator 27 with an output of 13,600 kWe. The exhaust gas from theturbine 23 drives the power turbine 22 which drives the low pressurecompressor 21 from which the low pressure air is supplied to the highpressure compressor 25 through an intermediate cooler 28. In said coolerthe heat of compression is recovered e.g. for preheating the feed waterto the waste heat boiler 17.

The heat comsumption in the fluid bed 50 for the indirect heating of thecompressed air from the high pressure compressor 25 is substantiallylarger than the heat consumption in the pyrolysis beds 34. For obtaininga suitable (<30° C., preferably <10° C.) temperature decrease of the bedmaterial in the pyrolysis beds 34 the distribution of the bed materialbetween the separation steps 32 and 33 is according to the inventioncontrolled. Said control is achieved preferably by using in the "coarsestep" 32 a separating device which gives a degree of separation which istoo high, and in a controlled by-pass (not shown on FIG. 1) a requiredflow of suspension is by-passed to the "fine step" 33.

The heat content of the flue gas from the cyclones 33, which is almostfree from bed material, is used in the steam generator 30 and itseconomiser 46. After the final cleaning in the dust bag filter 47 theflue gases are expelled to the atmosphere 49 at about 120° C. after apressure increase in the blower 48.

The lower effect from the gas turbine unit 20 compared with the unit 10which is essentially identical therewith as regards its components, iscaused by the fact that the power turbine 22 of the unit 20 operateswith ˜400 mbars back pressure since the exhaust gas therefrom, about 86kg/sec (about 480° C.), is used as fluidizing gas (primary air) andsecondary combustion air in the combustion chamber 31 of the ACFCB steamgenerator 30, which gives an air surplus of e.g. 10%. A small partialflow of the exhaust gas from the power turbine 22 is used as fluidizinggas in the fluid bed 50, which operates at 0.6<ε<0.7, and the gas istherefrom passed to the combustion chamber 31.

The saturated high pressure steam (145 bars, 338° C.) from the steamgenerator 30 and the waste heat boiler 17 is superheated in the steamgenerator to 535° C. and is fed to the high pressure part 41 of thesteam turbine unit 40. About 85% of the exhaust steam therefrom 34 bars,328° C.) is resuperheated in the steam generator 30 to 535° C. and thepressure is decreased in the low pressure part 42 against a vacuum inthe condensor 44, whereas the rest, including a certain amount ofbleed-off steam from the low pressure part, is used for feed waterpreheating 45. The turbine generator 43 thereby gives an output of102,300 kWe.

With subtraction for feeder pumps etc. with 2,300 kWe the combined powerplant thus according to the example of the invention gives an output of130 MWe which amounts to a net efficiency of 45.6%--a substantiallyhigher efficiency than what can be achieved with the PFBC concept.

The coal on which the example is based comprises 37% of volatiles whichmeans that 23% of the electric power production can be performed in thegas turbine units 10 and 20. If coal with 40% of volatiles is used thepercentage of the electric power generated in the gas turbine unitsincreases to above 23% which means that the net efficiency will amountto >46%.

If a power plant operating according to the method of this invention isbased on coal with e.g. 40% of volatiles and the gas turbines 10 and 20resp. designed for this, and it were for some reason possible to supplythe plant only with coal with e.g. 30% of volatiles, the efficiencywould decrease and the gas turbine units could not be used at anoptimum. In such a case it is, although the oil calories are moreexpensive, economically motivated to add oil to the pyrolysis in aquantity corresponding to the volatile substances which are lacking inthe supplied coal.

A power plant according to this example gives in a per se known way adesired low release of SO₂ by adding a controlled flow of finely-dividedlime-stone to the combustion chamber 31 of the steam generator 30 whereCaO formed binds SO₂ as CaSO₄. For obtaining a desired low release ofSO₂ a controlled surplus of CaO is maintained in the bed materialrecirculation circuit to the steam generator 30.

The pyro gas 35 from the pyrolysis beds 34 is substantially free from Ssince the equilibrium

    CaO+H.sub.2 S⃡CaS+H.sub.2 O

is displaced very far to the right at 830° C. About 60% of the S whichis fed to the pyrolysis beds with the coal is present in the char, about7% in the tar and about 30% is bonded as CaS in the bed material leavingthe pyrolysis beds 34.

Since only 30% of the tar, containing about 2% of the S fed into thesystem with the coal, is used for reheating the exhaust gas from theturbine unit 10, 98% of the S supplied bonded to the char, tar and CaSis introduced into the combustion shaft 31 of the steam generator 30where it is combusted at 850° C. in the presence of CaO. This means thatthe power generation according to the invention requires marginallylarger addition of finely-divided lime-stone compared with conventionalACFBC for obtaining the same low SO₂ release in the expelled flue gas.

The formation of NOx in the combustion of fossile fuels increasesheavily with the combustion temperature but also with the concentrationof O₂. Thus, for example, the content of NOx in the flue gas from thecombustion of coal powder in a burner of the multi-step type amounts to400-500 mg NOx/Nm₃ with an air excess of 50% which is normal for powderfiring. With a carefully operated ACFBC <80 mg NOx/NM³ can be maintainedbecause of the low combustion temperature (850° C.) and the low excessamount of air required for a high burn-out (about 99%).

Since the formation of NOx in ACFBC to >50% originates from the contentof organically bonded N in the fossile fuel and about 35% thereof isexpelled as N₂ and NH₃ in the pyrolysis, the supply of organicallybonded N to the combustion chamber 31 of he steam generator 30 is lowerin the power generation according to this invention, which gives theresult that the content of NOx in the flue gas will remain at <60mg/Nm³.

According to the example of this invention about 12% of the generatedelectric power is obtained in the gas turbine unit 10 which uses cleanedpyro gas as fuel. The exhaust gas 16 from the unit 10 comprises about300 mg NOx/Nm, if no particular measures are taken. About 80% of thedischarge of NOx can, however, easily be eliminated in a per se knownmanner by adding NH₃ in the molar ratio NH₃ NOx ˜0.8 to the 360° C. hotexhaust gas 16 and passing the gas mixture over a per se known catalyst.The exhaust gas thereafter will contain about 60 mg NOx/Nm³ at an excessof about 5 mg NH₃ /Nm³.

The NH₃ obtained in the pyrolysis of the fossile fuel is resent in thetar-water phase from the cleaning 36 of the raw pyro gas and can easilybe recovered therefrom and used for the denoxing operation mentionedabove. According to the example about 15 g NH₃ /sec.cante recovered fromthe tar water. The denoxing operation treatment of the exhaust gas 16requires about 2/3 thereof and the rest is sufficient for decreasing theNOx content in the flue gas from the ACFBC steam generator 30 from about60 to 10-15 mg NOx/Nm³, if desired.

A special advantage of the addition of NH, to the flue gas from theACFBC steam generator 30 is that the content of N₂ O in the flue gas,which according to the latest results is substantial with ACFBC, isdecreased substantially. This is essential since N₂ O contributes to theso-called green-house effect in the same high degree as CFC(chlorofluoro carbons) and also to the decomposition of the ozon layer,wherein the relative importance of N₂ O compared with CFC is 1:4.

If in a per se known manner the combustion chamber 13 of the gas turbineunit 10 is supplied with steam, e.g. bleed-off steam at 15 bars from thelow pressure part 42 of the steam turbine unit 40, at a flowcorresponding to 0.07 kg/Mca supplied fuel, the NOx content in theexhaust gas 16 will decrease to about 150 mg/Nm³ and simultaneously theoutput effect increases with about 2.5%. Since the output effect fromthe low pressure part 42 of the steam turbine 40 simultaneouslydecreases somewhat because of the bleed-off, the net effect increasewill be about 1%. The addition of NH₃ to the exhaust as 16 in theproportions mentioned above causes in this case a decrease of the NOxcontent therein to about 30 mg/Nm³, and the average NOx content in thetwo flue gas flows from the power plant according to the example afterdenoxing with the aid of internally produced NH₃ will amount to <20mg/Nm³ --a particularly low and environment-friendly level.

As a summary, the electric power generation based on solid fossile fuelsaccording to this invention gives the following substantial advantages:

a very high net efficiency which for 40% of volatiles in the coal feedexceeds 46%,

a desired low discharge of SO₂,

a low NOx discharge or a very low discharge if internally produced NH₃,is used for denoxing in a per se known manner.

We claim:
 1. An environment-friendly process for generating electricalenergy with very high efficiency on the basis of combustion of solidfuels, selected from the group consisting of fossil fuels and othersolid fuels containing over 20% of volatiles, in an ACFBC steamgenerator combined with two gas turbine units and a steam turbine, unit,wherein said solid fuel (60) is first pyrolyzed in a finely-divided formin contact with hot bed material in a fluid pyrolysis bed (34) throughwhich flows a controlled partial flow of said hot bed material from thecirculating flow of said ACFBC steam generator (30), which partial flowmixed with formed char is returned to a combustion chamber (31) of saidsteam generator (30), in which chamber said char acts as a fuel, whereaspyro gas is (35) formed, after cleaning (36) contaminants such as dustor tar from said pyro and is used as a fuel on one of said gas turbineunits (10), an exhaust gas (16) from said unit being used for generatingsteam (17), having a high pressure compressor (22) provide compressedair, heating said compressed air partially and indirectly in a heatexchanger (51), and operating said other gas turbine (20), said heatexchanger submerged in a classical type fluid bed (50) through whichflows a controlled partial flow of said hot bed material from thecirculating flow circuit of said ACFBC steam generator (30), finallyheating of said compressed air is performed by burning directly therein(24) gaseous or liquid fuel, operating said turbine unit with a backpressure provided by said exhaust gas from a power turbine part (22) ascombustion air in said ACFBC steam generator (30) for generating thereinsuperheated high pressure steam for said steam turbine unit (40).
 2. Aprocess according to claim 1, wherein said bed material is separatedfrom a hot flue gas suspension, of said bed material, leaving the top ofsaid combustion chamber (31) of said ACFBC steam generator (30) in twosteps: a first "coarse step" (32) wherein the content of bed material insaid flue gas suspension is decreased by 60-90%, and a "fine step" (33)wherein the content is reduced to less than 3% of the original contentof said bed material in said suspension, said material from said "coarsestep" (32) being introduced below the bed surface in said fluid bed (50)comprising the heat exchanger (51) and returned through overflows to thelower part of said combustion chamber (31) of said steam generator (30),said fluid bed (50) being fluidized with the aid of a small partial flowof said exhaust gas from said power turbine (22) of said back pressuregas turbine (20), said exhaust gas (53) from said fluid bed (50) beingsupplied to the combustion chamber (31) of said steam generator (30),and material from the "fine step" (33) is supplied to the fluidizedpyrolysis bed (34) and from there returned to the lower part of thecombustion chamber (31) of the steam generator (30) through an overflow.3. A process according to claim 1, wherein said finely-divided solidfuel is injected into the lower part of said pyrolysis bed (34), saidinjection being performed with the aid of a gas which is essentiallyinert against the fuel, whereas the bed is stirred with gas of the sametype and wherein raw pyro gas (35) produced in the bed is removed fromthe top thereof.
 4. A process according to claim 1, wherein NH₃ formedin the pyrolysis of said solid fuel is recovered and that a controlledflow thereof is used for denoxing said exhaust gas from said pyro gasfired gas turbine unit (10), and for denoxing the flue gas from saidACFBC steam generator (30).
 5. A process according to claims 1, whereinoil is supplied to the pyrolysis bed (34) as a complement to thevolatile substances in the coal feed.
 6. A process according to claim 2,wherein said finely-divided solid fuel is blown into the lower part ofsaid pyrolysis bed (34), said injection being performed with the aid ofa gas which is essentially inert against the fuel, whereas the bed isstirred with gas of the same type and wherein raw pyro gas (35) producedin said bed is removed from the top thereof.
 7. A process according toclaim 2, wherein NH₃ formed in the pyrolysis of said solid fuel isrecovered and that a controlled flow thereof is used for denoxing saidexhaust gas from said pyro gas fired gas turbine unit (10), and fordenoxing said flue gas from said ACFBC steam generator (30).
 8. Aprocess according to claim 3, wherein NH₃ formed in said pyrolysis ofsolid fuel is recovered and that a controlled flow thereof is used fordenoxing said exhaust gas from said pyro gas fired gas turbine unit(10), and for denoxing said flue gas from the ACFBC steam generator(30).
 9. A process according to claim 6, wherein NH₃ formed in saidpyrolysis of solid fuel is recovered and that a controlled flow thereofis used for denoxing said exhaust gas from said pyro gas fired gasturbine unit (10), and for denoxing the flue gas from the ACFBC steamgenerator (30).
 10. A process according to claim 2, wherein oil issupplied to the pyrolysis bed (34) as a complement to the volatilesubstances in said solid fuel feed.
 11. A process according to claim 3,wherein oil is supplied to the pyrolysis bed (34) as a complement to thevolatile substances in said solid fuel feed.
 12. A process according toclaim 4, wherein oil is supplied to the pyrolysis bed (34) as acomplement to the volatile substances in said solid fuel feed.
 13. Aprocess according to claim 3, wherein oil is supplied to the pyrolysisbed (34) as a complement to the volatile substances in the coal feed.14. A process according to claim 6, wherein oil is supplied to thepyrolysis bed (34) as a complement to the volatile substances in saidsolid fuel feed.
 15. A process according to claim 7, wherein oil issupplied to the pyrolysis bed (34) as a complement to the volatilesubstances in said solid fuel feed.
 16. A process according to claim 12,wherein oil is supplied to the pyrolysis bed (34) as a complement to thevolatile substances in said solid fuel feed.
 17. A process according toclaim 9, wherein oil is supplied to the pyrolysis bed (34) as acomplement to the volatile substances in said solid fuel feed.
 18. Aprocess according to claim 1, wherein oil is supplied to the pyrolysisbed (34) as a complement to the volatile substances in said fossil coalfeed.
 19. A process according to claim 2, wherein oil is supplied to thepyrolysis bed (34) as a complement to the volatile substances in saidfossil coal feed.
 20. A process according to claim 3, wherein oil issupplied to the pyrolysis bed (34) as a complement to the volatilesubstances in said fossil coal feed.
 21. A process according to claim 4,wherein oil is supplied to the pyrolysis bed (34) as a complement to thevolatile substances in said fossil coal feed.